Bugfix - Make timestamp default a callable

Timestamps in EventLog were not recorded during event insertion time.
They were fixated when EventLog was initialized and kept duplicating
across eventlog records.

CHANGELOG.rst

@@ -1,6 +1,11 @@
 Release History
 ===============
 
+0.5.8
+-----
+
+* Fix Issue with timestamps in EventLog
+
 0.5.7
 -----
 

src/protean/infra/event_log.py

@@ -11,7 +11,7 @@
 class EventLog(BaseAggregate):
     kind = String(max_length=50, required=True)
     payload = Dict(required=True)
-    timestamp = DateTime(required=True, default=datetime.utcnow())
+    timestamp = DateTime(required=True, default=datetime.utcnow)
 
 
 class EventLogRepository(BaseRepository):

tests/event_log/test_event_log_aggregate.py

@@ -0,0 +1,6 @@
+import pytest
+
+
+@pytest.mark.skip
+def test_timestamps_are_logged_properly():
+    pass